"If @TedCruz doesn't clean up his act, stop cheating, & doing negative ads, I have standing to sue him for not being a natural born citizen," Trump warned Friday on Twitter.

Cruz was born in Canada to an American mother, and most legal experts believe that he meets the constitutional requirement that presidents are "natural-born" citizens.

But the courts have not formally ruled on the issue, and Trump has repeatedly raised doubts about his opponent's eligibility for the Oval Office. Trump frequently warns that Democrats would sue the senator if he got the nomination. In January, he jokingly floated the idea of suing Cruz himself.

The senator — who has repeatedly mocked and dismissed Trump's concern over his birthplace — launched an attack ad on Thursday that accused Trump of having "a pattern of sleaze stretching back decades." The commercial focused on Trump's support for eminent domain, the government's power to seize private property.